Adult canine heartworm (Dirofilaria immitis) reside in the right ventrical and the adjacent blood-vessels of the host dog. The female heartworm deposits microfilaria which enter the bloodstream. These microfilaria cannot further develop unless they are ingested by the intermediate host, which consists of various mosquito species. Within the mosquito, the microfilaria develope into the third stage (infective larvae). The larvae gain entrance to the dog upon refeeding by the mosquito and after a period of several months migrate to the heart where they mature into adults.
The condition is most prevalent in coastal areas but can occur anywhere with an indigenous mosquito population and infected dogs. Treatment of both the adult and microfilaria has proven to be very difficult.
Thiacetarsamide is a known agent for the treatment of adult canine heartworm (see Jackson Journal of the American Veterinary Medical Association 142, pages 23-26 (1963) and Jackson et al. Proceedings of the Heartworm Symposium--1980, pages 137-140, Otto, Editor, Veterinary Medicine Publishing Company (1981)). However the drug is not entirely efficacious and the percentage of adult worms killed following the normally prescribed two or three day regimen treatment may only range from 60 to 70 percent (see Jackson et al. Proceedings of the Heartworm Symposium--1980, pages 153-156, Otto, Editor, Veterinary Medicine Publishing Company (1981)).
Ivermectin and other related avermectin compounds are a new group of antiparasitic compounds of microbial origin and are described in U.S. Pat. Nos. 4,310,519 to Albers-Schonberg et al. and 4,199,569 to Chabala et al. Within the broad range of activities possessed by these compounds, they have been found to be very active against the microfilarial stage of canine heartworm. (See Blair et al. American Journal of Veterinary Research, 40, pages 1031-1032 (1979) and Campbell et al. Journal of Helminthology, 52, pages 308-310 (1978)). However, the avermectins have been found to be substantially ineffective against adult canine heartworm when used alone. (See Jackson et al. Supra, page 136).
It has been surprisingly found that the administration of ivermectin at 3 to 6 weeks after the administration of thiacetarsamide dramatically increases the activity of the combination against the adult worms.